Behold, I am God; give heed to my word, which is quick and powerful, sharper than a two-edged sword, to the dividing asunder of both joints and marrow; therefore give heed unto my word. (D&C 11:2)

This verse and similar language is used to introduce a number of revelations in the Doctrine and Covenants—sections 6, 11, 12, 14, 33.

Why does the Lord say His word is quick? 

I suspect that He doesn’t mean quick in the sense of speed, but in the old sense of “alive,” just like we are promised elsewhere that we will be quickened, which means we will be made alive.

Is the Lord’s word alive?

I think so. When I read it, it moves me and stirs me. It wakes me up and blows fresh air through my soul. There is health and vigor in it.

Why does the Lord say His word is also like a sword that divides asunder joins and marrow?

You only have to read a few of the revelations to see there is so much plain-speaking and honesty with no softening of the the truth that it cuts through all layers of rationalization and self-deception. 

But why say it can divide asunder joints and marrow? Why use that imagery instead of saying something like splitting hairs down the middle or piercing to the heart? 

To talk about cutting bone instead of cutting to the heart underlines the sharpness and strength of the word. (And maybe it says something about the stubbornness of the listeners and readers.)  Also, cutting joints makes me think of cutting limbs off, taking a cross-section in one way, while cutting through marrow makes me think of cutting down through the bone long-ways to reveal another kind of cross-section. That idea of being able to cut cross-sections in two different ways through very hard material encapsulates a great lesson about the power and precision and versatility of the Lord’s words.

Also, I notice there’s a contrast that is set up too. The word is something that can quicken and enliven, or it can harm what gets in the way.  The word of God does tend to do that, depending on the listener. One who listens and obeys is made more spiritually alive, while one who rejects it is gradually cut off from the Lord’s presence.